Ionization Approximation theory Helium Protons Atomic physics Atoms Laser pulses Ions Atomic ionization Electron energy levels Electrons Hydrogen Ionization of gases Kinematics Numerical solution Short laser pulse Born approximation Cross section Impact ionization Initial state Ionization probabilities Photoionization Photons Projectile charge Projectiles Total ionization cross sections Above-threshold ionization Angular distribution Antiprotons Atom lasers